Laptop starts but won't respond or shut down

.Please help! I am tearing my hair out with our laptop. Have had for almost five years and last week it suddenly stopped responding to even the smallest task, and I am unable to shut it down (I am holding down the Power Off button for a few seconds each time which I know is bad for the laptop.
I can load Windows but then anything I try to do is unresponsive, i.e. open amy programmes or files. I can open the Toshiba desktop assist and each time the Action Centre brings up a message saying 'finish installing device software' - I'm unable to access the Device Manager to see which device this could be referring to (could be a USB key or the mouse) but when I click install, the action centre changes to 'no action needed' until I force shut down and start pc again after which the message about "finish installing device software" returns again.

i feel like I could resolve this if I were able to reboot in safe mode but I am unable to do this (shift and F8 during start up simply doesn't work) and I am unable to change my user status to administrator as I can't open any of the settings.

Since the problems began, I have also had a watermark appear on bottom RHS of screen saying Windows 8.1 Build 9600 which some people suggest means there could be a problem with my WIndows 8.1 license or installation (I think the laptop came preloaded with Windows 8.1 - if there was an installation disc, it could be anywhere so I am unable to access a product key!)

To boot into safe mode from powered off state with Windows 8.1 try this:-
Traditionally one presses F8 (or Shift+F8) after Power ON to get to the screen where one could select Safe Mode boot option. However, my Windows 8.1 computer requires a different procedure and it may work on yours: -

Power ON and immediately press F10 a few times.
This brings up “Edit Boot Options” screen.

Press Esc once.
This brings up “Windows Boot Manager” screen.

Press F8.
This brings up “Advanced Boot Options”.

Scroll to required option (safe Mode/Safe Mode with Networking etc) and press Enter.
This should take you into your selected option.